Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user's total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data in assigned channels that correspond to specific unique codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, modulated by a 64-ary orthogonal modulation, and spread prior to transmission by the mobile station. The data symbols following interleaving may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation is found in the TIA IS-95 CDMA standard. Another implementation is the TIA S-2000 standard.
In order to increase the reliability of CDMA receivers, base stations and wireless terminals frequently transmit M copies of the same signal, staggered in time, to the other device. The receiving device typically uses multiple receive paths, such as in a rake receiver, to capture each of the copies. The captured copies are summed to produce a composite signal in order to improve the signal to noise ratio. This allows the composite signal to be more easily de-spread and recognized by a signal correlator or matched filter. However, this approach requires a large number of components and a large circuit area. Additionally, the repeated transmission of M copies of the same signal is wasteful of scarce bandwidth.
Furthermore, wireless digital communication systems increasingly are using multicarrier CDMA (MC-CDMA) and orthogonal frequency division multiplexing (OFDM) CDMA. In OFDM-CDMA, different wireless terminals (or mobile stations) are allocated different frequency spreading codes. The advantage of OFDM-CDMA is that the number of codes assigned to each wireless terminal is adjustable, leading to different data rates for different wireless terminals. However, the fact that each wireless terminal must transmit its signal over the entire spectrum leads to an averaged-down effect in the presence of deep fading and narrowband interference.
U.S. Pat. No. 6,683,908 to Cleveland disclosed an apparatus and a method that eliminate the need to transmit M copies of the same signal in order to improve signal reception. The teachings of U.S. Pat. No. 6,683,908 are hereby incorporated by reference into the present application as if fully set forth herein.
The apparatus and method of U.S. Pat. No. 6,683,908 eliminate the need to transmit M copies of the same signal by storing in memory an original copy of the received signal and generating pseudo-replicas using the stored samples of the original received signal. Each pseudo-replica is generated by randomly interchanging samples of the original received signal that occurred during time slots of the original received signal that correspond to Logic 1 and by randomly interchanging signal samples that occurred during time slots of the original received signal that correspond to Logic 0. The original signal and one or more pseudo-replicas are then combined to form a composite signal that has an improved signal-to-noise ratio (SNR).
The SNR is improved because noise in communication systems is not coherent and tends to cancel when the pseudo-replicas are repeatedly added together. But the signal is coherent and the signal components tend to add together as the pseudo-replicas are repeatedly added together. Thus, the SNR improves.
However, the apparatus and method of U.S. Pat. No. 6,683,908 operate on samples from time domain signals. This is not ideally suited for the frequency domain signals that are present in MC-CDMA and OFDM-CDMA receivers.
There is therefore a need in the art for improved multi-carrier CDMA communication systems that have an improved signal-to-noise ratio in the receiver. In particular, there is a need for multi-carrier CDMA communication systems that are not required to transmit multiple copies of a signal in order to improve SNR in the receiver.